1. Field of the Invention
This invention relates to a flat panel X-ray detector which is adapted to be used in a medical X-ray diagnostic device.
2. Description of the Related Art
In recent years, there is an increasing trend in the medical field to store the medical data of patients in the form of data base. The reason for this trend is due to the facts that since patients often desire to utilize a plurality of medical facilities for the medical treatment of the same disease, one of the medical facilities which is going to examine such patients may require to know the data that has been already obtained by other medical facilities, in order to enable the medical facilities to perform more appropriate medical treatments on such patients.
There is also a strong demand to store the image data of X-ray photographing in the form of data base, and therefore, it is desired to digitize the images produced by X-ray photography. In the conventional medical X-ray diagnostic device, X-ray images are taken by making use of a silver film. If it is desired to digitize such X-ray images, an operation of reading the images that have been photographed and developed on the film is required by making use of a scanner and the like, thus necessitating a troublesome and time-consuming operation.
Recently, there has been developed an image intensifier TV (II-TV) system which is capable of directly digitizing the X-ray images by making use of a CCD camera of about 1 inch and a large scale vacuum tube provided with a photoelectric film, accelerating electrodes and a fluorescent film. However, this II-TV system is accompanied with a problem that in the case of diagnosing lungs for example, an optical apparatus for achieving the convergence of light for taking a picture of as large an area as 40 cm×40 cm is required, thereby necessitating a large scale apparatus. Further, because of the distortion of image caused due to the deflection of electron beam resulting from the earth's magnetism, or because of the deterioration of resolution resulting from a series of electronic and optical systems such as a fluorescent film, a CCD, etc., the quality of image is caused to deteriorate.
There is also proposed, as a new system for overcoming the problems accompanied with the aforementioned conventional two systems, an X-ray image pickup apparatus using an amorphous silicone thin film transistor (a-Si TFT) (hereinafter referred to as a flat panel X-ray detector) (for example, U.S. Pat. No. 4,689,487).
According to this flat panel X-ray detector, the pixels thereof are all constituted by the a-Si TFT, a photoelectric conversion film and pixel capacitance, and these pixels are arranged in the form of an array where hundreds or thousands of pixels are aligned along vertical and horizontal lines (hereinafter referred to as TFT array).
In this flat panel X-ray detector, a bias voltage from a power source is applied to the photoelectric conversion film, and the a-Si TFT is connected with a signal line and with a scanning line and controlled to turn ON/OFF by means of a scanning line driving circuit. The terminal of the signal line is connected, via a change-over switch, with a signal-detecting amplifier.
When a beam of light is irradiated to the detector, an electric current is permitted to flow through the photoelectric conversion film so as to enable electric charge to be stored in the pixel capacitance. The scanning line is designed to be driven by means of the scanning line driving circuit, so that when all TFTs connected with a single scanning line are turned ON, the electric charge stored as mentioned above is transmitted, via the signal line, to the amplifier side. In every pixel, electric charge is inputted into the amplifier by making use of the change-over switch and then converted sequentially into a signal so as to enable it to be displayed on a CRT, etc. In this case, depending on the magnitude of the light entering into the pixel, the magnitude of electric charge is caused to change, thereby causing the output amplitude of the amplifier to change.
According to this system, X-ray images can be directly turned into digital images by A/D-converting the output signals of the amplifier. Further, since the pixel region thereof is formed of the same structure as that of a thin film transistor liquid crystal display (TFT-LCD) which is commonly employed in a note-type personal computer, a display which is thin and large in picture plane can be easily manufactured.
These conventional systems explained above are all related to a flat panel X-ray detector of indirect conversion system wherein the incident X-ray is converted by means of a fluorescent substance, etc. into visible light, which is then converted into electric charge by means of the photoelectric conversion film of each of the pixels.
Apart from these conventional systems, there is also known a flat panel X-ray detector of direct conversion system wherein the X-ray entering into the pixels is directly converted into electric charge. This flat panel X-ray detector conversion system differs from that of indirect conversion system in the respect that an X-ray is directly converted by means of the photoelectric conversion film into electric charge, which is then stored in the pixel capacitance. Namely, this flat panel X-ray detector of direct conversion system is almost identical in structure with the flat panel X-ray detector of indirect conversion system except that the fluorescent substance is not provided therewith.
This flat panel X-ray detector of direct conversion system is constructed such that a capacitor (Cst) consisting of a laminate structure comprising a capacitor electrode, an insulating layer and an auxiliary electrode, a switching TFT connected with the capacitor, and a protective TFT are disposed on the surface of a glass substrate. Each of these components is covered with a protecting film having a contact hole disposed over the auxiliary electrode. On this protecting film, there are successively superimposed a pixel electrode (which is connected via the contact hole with the auxiliary electrode), an X-ray-charge conversion film, and a common electrode (upper electrode). The pixels constructed as described above are arranged in a form of array.
When X-ray is irradiated to the detector, the X-ray is converted by means of the X-ray-charge conversion film into electric charge which is then accelerated by an electric field applied between the common electrode and the pixel electrode and stored in the capacitor. The switching TFT is driven through the scanning line so as to transfer the electric charge that has been stored in the capacitor to the signal line. The function of the protective TFT is to release electric charge whenever an excessive electric charge happens to be generated.
It is required that the signal charge generated from the X-ray is enabled to immediately reach the pixel electrode and stored in the pixel capacitance. If the signal charge is permitted to remain inside the X-ray-charge conversion film, it would lead to the generation of defective images such as residual images where the previous image patterns are left behind or the deterioration of resolution. Most of these defective images are caused to generate due to the signal charge that has been left remained in the X-ray-charge conversion film and acts to badly affect the traveling of the signal charge that has been newly generated by the X-ray. Further, there is another problem that when the X-ray-charge conversion film is accompanied with a large number of defects, electric current passing through such defects is generated, thereby increasing the dark current.
It is conventionally known to construct the X-ray-charge conversion film by making use of PbI2 which is excellent in properties and is expected to be very useful as a material for forming the X-ray-charge conversion film. As a matter of fact however, since this PbI2 is poor in crystallinity especially when it is formed into a thin film, the employment of this PbI2 is also accompanied with the same problems as mentioned above, i.e. residual images, the deterioration of resolution, the generation in considerable degree of dark current, etc. Therefore, up to date, no one has succeeded to manufacture the X-ray-charge conversion film which is excellent in properties (see R. A. Street et al., SPIE Vol. 3659, P. 36,1999).
It is imperative to improve the quality of the X-ray-charge conversion film in order to overcome the generation of aforementioned defective images. According to the prior art however, since a polycrystalline X-ray-charge conversion film which is not so good in quality is deposited on the surface of the underlying substrate in general, a large number of grain boundaries and traps are caused to exist in the film near the interface between the film and the substrate, thereby making it impossible to avoid the generation of residual images and the deterioration of the resolution of images. Furthermore, since a large magnitude of dark current is generated in the case of such a defective film as mentioned above, it is very difficult to perform the detection of images under a faint X-ray dose.
As explained above, as long as the conventional photosensitive films are to be employed for the manufacture of the flat panel X-ray detector, it has been very difficult to form the X-ray-charge conversion film which is excellent in quality. Therefore, it has been very difficult to overcome the generation of residual images, as the deterioration of the resolution of images, and especially the generation of large dark current.